1. Field of the Invention
The present invention relates to a developing device used to develop an electrostatic latent image formed on an image carrier in an image forming apparatus such as a copier, printer or the like.
2. Description of the Related Art
Various types of developing devices are conventionally used for developing electrostatic latent images formed on an image carrier in image forming apparatuses such as copiers, printers and the like.
One such developing device, for example, is shown in FIG. 5. A cylindrical developer-carrying member 111 provided with an internal magnet roller 111a is disposed opposite an image-carrying member 101 in body 110 of a developing device. A developer supplying member 112 such as a bucket roller or the like supplies developer 102 accommodated in body 110 to the surface of the developer carrier 111. Developer 102 is maintained on the surface of the developer carrier 111 by the magnetic force of the magnet roller 111a, and is transported via the rotation of the developer carrier 111. A regulating member 113 regulates the amount of developer 102 transported to the region at which developer carrier 111 confronts the image carrier 101 (hereinafter referred to as "developing region"). The developer 102 regulated by regulating member 113 is supplied to the developing region, and develops an electrostatic latent image formed on the image carrier 101.
As shown in FIG. 5, a direct current (DC) and alternating current (AC) from DC power source 114 and AC power source 115 are superimposed and supplied to developer carrier 111, so as to generate an electric field in which an AC electric field is superimposed on a DC electric field in the developing region to accomplish development. This electric field is effective for developing an electrostatic latent image formed on image carrier 101, and prevents disruption of a toner image formed on image carrier 101 by a magnetic brush of developer 102.
When a halftone image is developed by a method using the action of an electric field comprising an AC electric field superimposed on a DC electric field in a developing region, there is marked variation of image density due to variations in surface potential of image carrier 101 relative to the halftone image, which disadvantageously produces irregular density in the image so as to prevent obtaining images having excellent balance. Images formed in this way also lack adequate texture.
In recent years it has been proposed, as disclosed in U.S. Pat. No. 4,610,531, to utilize the action of an electric field comprising an AC electric field superimposed on a DC electric field in a developing region by alternatingly repeating a first time period t1 (action period) during which an AC voltage is applied between a developer carrier 111 and an image carrier 101 and a second time period t2 (rest period) during which said AC voltage is not applied so as to provide a pair of AC electric field of opposite direction in first action period t1, and use the final component of said AC electric field as an electric field component normally to pull back developer 102 to the developer carrier 111 when accomplishing development.
In this instance, however, developer 102 continues to be pulled back to the developer carrier 111 even during the second action period t2 when the said second action period t2 is entered after the AC electric field stops in the state in which said developer 102 is normally pulled back to the developer carrier 111 from the image carrier 101 while said developer 102 is traveling between said image carrier 101 and developer carrier 111. Therefore, when the AC electric field is subsequently applied, the developer 102 is inadequately supplied to the image carrier 101, thereby reducing the image density of the formed image, and preventing the production of images having suitable density in the case of high-speed developing.